The invention relates to a method of forming the radiation pattern of a high efficiency active antenna for electronically-scanned radar, and to an antenna implementing the method.
Electronic scanning greatly increases the performance of radars, by virtue of its flexibility (number of possible operating modes) and its speed (quasi instantaneous beam shifting).
However the main drawback thereof is the very high number of phase control circuits required and often the very high number, of amplitude control circuits also required, thereby giving rise to losses, expense, mass, and power consumption which are often prohibitive.
Drawbacks relating to losses, expense, mass, and bulk have been overcome by mass-producing monolithic microwave integrated circuits (MMIC) on gallium arsenide. It is thus possible to make active transmit-receive (TR) modules very compactly which integrate the functions of phase-shifting, switching, transmission and reception, and amplification.
However, transmission power amplifiers made in MMIC technology have relatively low efficiency, and in addition their efficiency falls off if the output power level is varied.
In conventional designs, such variation in power level is essential both:
as a function of position to form a radiation pattern having low side lobe level (SLL); and
as a function of time in order to modulate lobe width to adapt it to the mission.
As a result, the power consumption of this type of active radar antenna is prohibitive.
There exist several documents in the state of the art and in particular:
parts I and II of the article entitled "Array radars: an update", by Eli Brookner published in Microwave Journal (Feb. and Mar. 1987);
the article entitled "Applicability, availability, and affordability of GaAs MMICs in military systems" by Eugene H. Gregory, published in Microwave Journal (March 1987); and
the article entitled "Affordable MMIC designs for phased arrays" by Ronald J. Naster, Anthony W. Jacomb-Hood, and Mark R. Lang, published in Microwave Journal (March 1987).
The first prior art electronically-scanned radars used diode or ferrite phase shifters for controlling beam depointing:
the major drawback of diode phase shifters is significant losses (several dB for 4/5-bit phase shifters), thereby requiring the already-critical power of the amplifiers to be increased; and
although ferrite phase shifters have losses of less than 1 dB, they suffer from significant mass and bulk. These parameters become critical with airborne radars, and they prevent such radars being mounted on satellites.
An important advance was made when monolithic microwave integrated circuits (MMICs) on gallium arsenide started to be mass produced. This technology makes it possible to manufacture various types of microwave circuit having very low mass and bulk, at relatively low cost, and in mass production quantities, and in particular it can be used for manufacturing controllable attenuators and phase shifters.
The major drawback of MMIC phase shifters relates to significant losses (more than 5 dB for a 0.degree.-360.degree. C. phase shifter having analog control or 4/5-bit digital control). However this aspect is secondary when these phase shifters are associated with amplifiers:
either high power amplifiers (HPAs) situated downstream from transmit phase shifters, since the losses then take place at low level and have no effect on limiting the output power of the amplifiers, it merely being necessary to increase the gain of the output amplifiers a little;
or low noise amplifiers (LNAs) situated upstream from the receive phase shifters, since providing the gain of these amplifiers is adequate (20 dB to 30 dB), then the losses inherent to the phase shifters have substantially no deleterious effect on the noise factor of the receiver.
Transmit-receive (TR) modules are generally manufactured on a common (alumina) substrate by connecting together a plurality of gallium arsenide chips each performing an elementary function. These chips are themselves mass produced using doping (diffusion or ion implanting), masking, oxidizing, . . ., techniques based on those used for making logic integrated circuits on silicon. Silicon ICs have shown their capacity for reducing cost enormously, without loosing reliability.
By connecting together several hundreds or thousands of such MMIC-TR modules in an active radar antenna (called "active" because it includes active devices in the form of amplifiers), it is possible to reconcile the requirements of electronic scanning with cost, mass, and bulk, which are critical parameters for airborne radars and even more important for space radars.
The final critical parameter for such active radar antennas is their DC power consumption.
The added power efficiency of HPA amplifiers: EQU .eta.a=(Pout-Pin)/PDC
is much lower in MMIC technology (by 15% to 20%) than in travelling wave vacuum tube technology which lies in the range 30% to 60% depending on the microwave waveband.
Efficiency is particularly poor when using class A (linear) HPAs while varying the input power and thus the output power: power consumption is determined by the bias currents and voltages which are set for the maximum Pout to be delivered. The same amount of power is consumed when Pin is reduced to reduce Pout.
An alternative consists in reducing bias voltages when a lower Pout is required. Power consumption is thus reduced, but considerably less than power output (in percentage or in dB). Efficiency .eta.a is thus significantly reduced.
However if high performance radiation patterns are to be formed, it is necessary:
at least to have one different Pout per TR module, so as to obtain the weighting required for illumination taper; and
in some cases where the mission requires a lobe of variable width, it is also necessary to vary the illumination taper amplitude law as a function of time, thus requiring the Pout of the HPAs to be varied.
As a result, active radar antennas have hitherto confronted the following dilemma: a radiation pattern having low side lobe levels, and preferably also being capable of being modulated, can only be obtained by reducing the efficiency of the distributed HPAs.
The resulting increase in power consumption has so far restricted the generalization of active antennas for airborne radar applications, and even more for space radar applications where the available power is very limited.
An object of the present invention is to escape from this performance/power consumption dilemma by providing a method of forming the radiation pattern of a radar antenna which is particularly well suited to active antennas (i.e. antennas having distributed modules including transmit and receive amplifiers).